A gerotor motor typically includes a housing defining inlet and outlet ports and a gerotor gear set defining expanding and contracting volume chambers. The motor further includes a valve arrangement to provide fluid communication between the ports and the volume chambers of the gerotor gear set. In certain cases, the gerotor gear set includes an externally-toothed star member that rotates and orbits around an internally-toothed ring member.
In certain gerotor motors, a gerotor motor has a two-piece valve drive including a main drive shaft and a valve drive shaft, and a valve arrangement disposed behind the gerotor mechanism opposite to the output shaft. In this configuration, the main drive shaft is used to transmit torque from the orbiting and rotating star member to a rotating output shaft, and the valve arrangement can be driven at a speed of rotation of the star member by means of the valve drive shaft which is in splined engagement with both the valve arrangement and the star member, as disclosed in U.S. Pat. No. 4,992,034.
In the gerotor motor employing such a two-piece valve drive, a portion of an axial length of the spines defined by the star member is required to drive the valve drive shaft to rotate and orbit as the star member rotates and orbits. Therefore, the spline connection between the star member and the drive valve shaft takes up a significant portion of the splines of the star member.